(1) Field of the Invention
This invention pertains to a pulley commonly used in consumer appliances, such as washing machines and dishwashers. Specifically, the invention pertains to a novel design of the pulley and a method of making the same.
(2) Description of the Related Art
Pulleys are commonly found in many applications where the rotary motion between two shafts is coupled together through a pulley and belt system. Many consumer appliances, such as washing machines and dishwashers, commonly have several pulleys in their drive systems to transfer the rotary motion of the drive motor to pumps, transmissions, and other devices requiring power coupled from the motor. Because of mass production of these consumer appliances, the number of pulleys required in production is also relatively high. In order to reduce costs of production, manufacturers have focused their cost savings efforts on large volume items such as pulleys where small improvements are often multiplied through volume into considerable savings.
Conventionally, pulleys in these applications are typically metal and machined to the desired geometry. Generally, bar stock is the raw material for such production and through sequential turning, boring, grooving, and cut-off operations, the pulleys are produced. However, pulleys produced from this type of machining are expensive since the machining operations for forming a pulley generally have long cycle times and tend to generate high percentages of scrap. In forming a pulley in this manner, a large majority of the bar stock material must be machined away to achieve the final desired pulley shape and design. Generally speaking, this process does not efficiently use the raw material since a large portion of the material is machined away as chips. The center hub of the pulley must be precision formed so that it may fit on the motor shaft with minimal slip and rotate about the motor shaft with minimal run out. Often this requires secondary operations and increased inspection to ensure the required levels of concentricity and dimensional accuracy are met. Additionally, bar stock is a relatively expensive form of material, and pulleys formed from bar stock tend to be rigid such that they are harder to install in the appliance.
Some manufacturers have attempted to use plastic pulleys in these applications because plastic raw materials are less expensive than metal forms of material and plastic pulleys may be formed in low cost, high speed, injection molding processes. However, plastic pulleys formed in this manner often have strength and wear problems in these applications such that their use is limited.
In order to decrease the number of manufacturing operations associated with the production of conventional metallic pulleys, manufacturers have produced pulleys as three-piece assemblies. In this design, the pulley has a center hub formed from a sintered powdered metal material. The hub has a precision center hole formed in it to allow the hub to be mounted on the shaft. To form the body of the pulley, circular pieces of sheet stock typically are stamped to create a general concave form. The two circular pieces of sheet stock are arranged side by side on the hub. The pieces are secured to the hub commonly through a brazing operation. The use of stamped flat sheet stock blanks eliminates much of the waste previously found when machining bar stock.
Although the three-piece pulley has many advantages over the machined pulley, the three-piece pulley also has many drawbacks. The powdered metal sintered hub is expensive to form and the center hole must be extruded and reamed to the required dimensions to fit on the shaft with minimal slip. Similarly, the stamped pieces must be attached to each other around the hub in a concentric arrangement with the hub to minimize circular run out therewith. Additionally, the assembly operation associated with arranging the pieces around the hub is time consuming and costly, and an inventory of circular pieces and hubs must be maintained in the proper ratios to maintain efficient production on assembly lines.
To further decrease the number of manufacturing operations associated with the production of conventional metallic pulleys, manufacturers have produced pulleys as two-piece assemblies. In this design, two circular metallic plates are each stamped into the configuration of one half of a pulley assembly. Each plate when stamped is formed with one half of the cylindrical center hub of the pulley, a flat annular connection surface surrounding the center hub and projecting radially outwardly from the center hub, and an annular angled drive surface surrounding the connection surface and oriented at an angle relative to the connection surface giving the stamped half of the pulley a general dish shape. The two pulley halves are brought together by connecting their connection surfaces. Various different means were employed in connecting the connection surfaces of the two stamped pulley halves, including mechanical connectors such as rivets, bent tab in slot connections, and clinch joints. The two stamped halves of the pulley have also been joined together by welding.
However, since the filing of the parent application it has been observed that many of the mechanical fastener connections between the two stamped pulley halves do not work well in an environment where the pulley is rotated by a belt wrapped around the pulley. The tension on the belt wrapped around the pulley exerts an axial force component against the pulley drive surfaces that pushes the two pulley halves apart from each other. Where the two pulley halves are attached by mechanical fasteners, and in particular where the fasteners are tabs formed on each of the pulley halves that are inserted through slots in the other pulley half and bent over, the axial force tends to straighten the bends in the tabs causing the two pulley halves to be separated from each other resulting in a failure of the pulley.
What is needed is a metallic pulley which may be manufactured by a cost effective method. Such a pulley would be assembled in fewer operations, require fewer parts, and use less expensive materials. Such a pulley would be manufactured with less scrap and quality issues than those pulleys in the prior art. Moreover, such a pulley would be more easily installed in the appliance. These benefits would result in a lower cost pulley which would reduce the overall cost of manufacturing the appliance.